The system studied is the ability of 8-day embryonic chick liver to exhibit adhesive recognition. (Liver cells adhere preferentially to other liver cells as opposed to a foreign cell type, i.e., neural retina cells). The problems of isotopically labeling cells, dissociating the organs into single cell suspensions, aggregating the dissociated cells, and collecting the labeled single cell suspension by unlabeled tissue aggregates have been examined. These studies have led to a marked improvement in an assay for measuring tissue specific adhesion. The assay can now be used to probe the molecular basis for cell/cell recognition. The adhesion of liver cells is dependent on cell surface adhesion receptors (proteins or glycoproteins) and on metabolic energy. No protein synthesis is necessary for tissue specific cell/cell recognition. The nature of the receptor molecules and the energy dependent step are currently being investigated. We are currently attempting to extend the cell adhesion studies to liver cell from other embryos (mouse and rat), adult animal liver cells, regenerating liver cells, malignant hepatoma cells, and to "normal" liver cells in tissue culture.